The present invention relates to diaphragm pressure sensors, and more particularly, to silicon-on-silicon differential input sensors with integrated signal conditioning circuitry and the process for producing them.
Sensors having a reduced thickness silicon diaphragm affixed to a glass wafer are known in the art. U.S. Pat. No. 4,543,457 issued to Petersen et al. on Sep. 24, 1985 ("the Petersen patent") discloses such a sensor. Sensors of this type may be used to measure a variety of parameters, such as pressure, temperature, acceleration or humidity. Silicon diaphragm sensors are reliable and may be fabricated at low cost. The output of such a sensor changes as the diaphragm is deflected by the condition sought to be measured.
A particular type of sensor uses variable capacitance. In variable capacitance sensors, the diaphragm is spaced apart from a confronting electrical plate. The capacitance between the diaphragm and the confronting plate changes in response to the deflection of the diaphragm. The usefulness of a given sensor is determined in part by the range of conditions over which the change in capacitance remains linear with respect to the change in input condition. Superior performance has been obtained with diaphragms of corrugated and bossed construction. These diaphragms deflect while remaining substantially parallel with the confronting contact, minimizing the undesirable effects of diaphragm curvature on sensor accuracy. Corrugated diaphragms have much larger regions of linearity than diaphragms without corrugations.
A major difficulty with conventional silicon-on-glass sensors of the type shown in the prior art is the difference between the coefficients of thermal expansion of silicon and glass. The silicon diaphragms are typically bonded to glass wafers by electrostatic bonding methods. Sensor inaccuracies result due to temperature changes because the silicon diaphragm and the glass wafer have different coefficients of expansion. This affects the geometric configuration of the device, resulting in degraded measurement accuracy. This problem could be avoided by mounting the silicon diaphragm on a wafer of similar silicon material; however, stray capacitances are introduced into the system when the diaphragm is bonded to silicon material.
Such stray capacitances present serious problems because they obscure accurate measurement of the capacitance between the diaphragm and the confronting electrode on the silicon wafer, effectively rendering measurements from the sensor useless. Historically, the thermal expansion problem, which arises from mounting the silicon diaphragm on glass, has been perceived by those skilled in the art as the least troublesome of the two. On the other hand, if the stray capacitance problem could be solved, silicon-on-silicon configurations free from thermal expansion problems could be produced.
Another problem with the silicon-on-glass construction is that it precludes the use of on board, integrated electronics for converting the output of the sensor into meaningful form and transmitting it to external electrodes. Instead, the circuitry must be produced separately and affixed to the glass wafer at a later time. These "hybrid" sensors are costly and inefficient to manufacture. Prior attempts to create one-piece silicon-on-silicon sensors have failed because the micro-electronics operations necessary to deposit integrated circuitry on the silicon are incompatible with the process of micro-machining operations necessary to create the diaphragm. No suitable solution to these problems has heretofore been found.
Accordingly, it is an object of the invention to provide a silicon-on-silicon capacitive sensor.
It is another object of the invention to provide such a sensor having monolithic integrated circuitry.
It is a further object of the invention to provide such a sensor having durable construction and capable of being produced at low cost.
These and other objects of the invention will become apparent to those skilled in the art when the following detailed description of the invention is read in conjunction with the accompanying drawings.